Third generation photovoltaics encompass a variety of designs including liquid-based dye sensitized Gråtzel solar cells (“DSSCs”), heterojunction solar cells, Förster resonance energy transfer (“FRET”) based solar cells, and organic solar cells. Liquid-based DSSCs utilizing a dye-coated layer of TiO2 nanoparticles have reached photoconversion efficiencies of over 11%, as described in M. K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, Combined Experimental and DFT-TDDFT Computational Study of Photoelectrochemical Cell Ruthenium Sensitizers, J. Am. Chem. Soc. 2005, 127, 16835-16847. Solid-state organic solar cells are easy to handle and manufacture, and have demonstrated efficiencies ranging from 2 to 8.13%, as described in S. H. Park, A. Roy, S. Beaupré, S. Cho, N. Coates, J. S. Moon, D. Moses, M. Leclerc, K. Lee, A. J. Heeger, Bulk heterojunction solar cells with internal quantum efficiency approaching 100%, Nature Photonics 3 (2009) 297-302. Another third generation photovoltaic technology is quantum dot (“QD”) that has reached photoconversion efficiencies of approximately 5%, as described in X. Wang, G. I. Koleilat, J. Tang, H. Liu, I. J. Kramer, R. Debnath, L. Brzozowski, D. A. R. Barkhouse, L. Levina, S. Hoogland, E. H. Sargent, Tandem colloidal quantum dot solar cells employing a graded recombination layer, Nature Photonics 5 (2011) 480-484. It is expected that photovoltaics utilizing multiple exciton generation (“MEG”) can be achieved with photoconversion efficiencies of over 30%, as described in A. Luque, A. Marti, A. J. Nozik, Solar cells based on quantum dots: Multiple exciton generation and intermediate bands, MRS Bulletin 32 (2007) 236-241.
For all such 3rd generation applications, cell designs that allow the direction of light absorption to be orthogonal to the direction of charge separation would offer significantly improved performance. Moreover, light absorption can be increased as desired without a penalty in charge recombination if such cells are comprised of crystalline structural elements that facilitate charge transfer, since the crystalline structures, in particular crystalline anatase TiO2, rapidly transport the charge to the underlying electrical contact without unwanted recombination of the photogenerated charge. Thus, for integration into DSSC, QD, MEG and other third generation photovoltaic designs, single-crystal 1-D anatase material architectures that are vertically oriented from a transparent conductive layer integral to an underlying glass substrate are desirable.
Most common transparent conductive layers are made of a transparent conductive oxide (“TCO”) such as tin oxide (“SnO2”), indium tin oxide (“ITO”) or fluorine-doped SnO2 (“FTO”). Self-organized TiO2 nanowire/nanotube arrays vertically oriented on a TCO substrate have been described and offer numerous benefits, such as large surface area for dye sensitization, resulting in enhanced light harvesting, easy transfer of electrons injected from photon-excited dye, vectorial (directed) charge transport to the electrical contact, and a readily assessable space for intercalation of the redox electrolyte or p-type semiconductor. However, while self-assembled vertically oriented 1-D titania nanoarchitectured films offer great potential for enhancing third generation photovoltaic efficiencies, to date such films have not been achieved. Something is always lacking in such fields, be it poor crystallinity of the electron transporting backbone, which results in poor charge transport; the wrong material phase, for example, rutile instead of anatase; damage to the transparent conductive oxide coating, which increases the series resistance and/or reduces film transmission; or low surface area, which limits the amount of light that can be absorbed.
The fabrication of polycrystalline anatase TiO2 nanotube arrays on FTO-coated glass of desired pore size and length by anodic oxidation of a sputter-deposited Ti film, has been described in G. K. Mor, O. K. Varghese, M. Paulose, and C. A. Grimes, Transparent Highly-ordered TiO2 Nanotube-arrays via Anodization of Titanium Thin Films, Advanced Functional Materials 15 (2005) 1291-1296. Fabrication of TiO2 nanotube arrays on FTO-coated glass up to 53 μm in length have been achieved, as described in O. K. Varghese, M. Paulose, C. A. Grimes, Long vertically aligned titania nanotubes on transparent conducting oxide for highly efficient solar cells, Nature Nanotechnology 4 (2009) 592-597. The as-anodized nanotubes are amorphous. They are crystallized by annealing in oxygen at elevated temperatures; the walls of crystallized nanotubes are anatase, while any residual Ti layer underneath the nanotubes will convert to rutile, as described in Varghese, O. K.; Gong, D. W.; Paulose, M.; Grimes, C. A; Dickey E. C., Crystallization and high-temperature structural stability of titanium oxide nanotube arrays, J. Mater Res 18 (2003) 156-165; and, M. Paulose, K. Shankar, H. Prakasam, S. Yoriya, O. K. Varghese, C. A. Grimes, Fabrication Of Highly-Ordered TiO2 Nanotube-Arrays of Great Length, U.S. patent application Ser. No. 16/693,123, filed Jul. 26, 2007. TiO2 nanotube arrays grown on FTO glass need to be annealed at relatively low temperatures, no more than ≈470° C., to minimize damage to the FTO layer, as described in Varghese et al. Annealing promotes diffusion of residual Ti atoms into the FTO layer and, perhaps, fluorine into the TiO2 layer, with a corresponding loss in electrical conductivity. The low annealing temperature prevents oxidation of residual Ti atoms, so film transparency suffers in progressively longer nanotube array films. Further, the closed end of the tubes makes it difficult to fill them with a viscous polymer. Thus while the nanotube arrays were of great scientific interest they did not transform third generation photovoltaics since the films were difficult to make, and faced a trade-off of poor crystallization or high series resistance.
The hydrothermal growth of vertically aligned TiO2 single-crystal rutile nanowire films on FTO-coated glass, has been described in X. Feng, K. Shankar, O. K. Varghese, M. Paulose, T. J. Latempa, C. A. Grimes, Vertically Aligned Single Crystal TiO2 Nanowire Arrays Grown Directly on Transparent Conducting Oxide Coated Glass: Synthesis Details and Applications, Nano Letters 8 (2008) 3781-3786; and, X. Feng, Karthik Shankar, C. A. Grimes, Single-Crystal Nanowires and Liquid Junction Solar Cells, U.S. Patent Application No. 2010/0139747. The self-assembled hydrothermally grown nanowires are of a highly crystalline rutile structure with preferred [001] orientation. Nanowire length is increased by the use of extended hydrothermal reaction times, but extended reaction times may also result in a thicker oxide layer at the nanowire base reducing the surface area. Use of the rutile nanowires in DSSCs gave a relatively low photoconversion of approximately 5% for highly optimized devices. In comparison to rutile, use of anatase in DSSCs results in a higher photovoltage due to the higher conduction band level of anatase. Further, for equal degrees of crystallization the charge transport properties of anatase are superior to those of rutile. Therefore, to facilitate fabrication of high performance 3rd generation photovoltaics, it is desirable to provide an improved photovoltaic application of self-assembled anatase TiO2-based 1-D nanostructures.
In 1999, Kasuga et al. (T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Titania nanotubes prepared by chemical processing, Advanced Materials 11 (1999) 1307-1311) reported the synthesis of needle-shaped (length ≈100 nm, diameter ≈8 nm) anatase TiO2 nanotubes through the mixture of anatase or rutile powders with an aqueous solution of 5-10 M NaOH at 110° C. for 20 h. A centrifuge was used to separate the nanotube powder from the solution, which was then washed with deionized water and subsequently immersed in a 0.1 M HCl solution for substitution of the sodium ions with the hydrogen ions. The hydrogen protons were then removed as water vapor through a high temperature anneal resulting in the needle-shaped anatase nanotubes.
In 2004, Wang et al. (W. Wang, O. K. Varghese, M. Paulose, C. A Grimes, Q. Wang, and E. C. Dickey, A Study on the Growth and Structure of TiO2 Nanotubes, J. Materials Research 19 (2004) 417-422) reported fabrication of randomly oriented single crystal anatase nanotubes, having diameters of 8 nm to 10 nm and lengths ranging from approximately 0.1 μm to 1 μm, from a NaOH solution in which anatase nanoparticles were dispersed. X-ray diffraction studies showed the structure of the as-prepared nanotubes to be the same as that of the starting anatase TiO2 nanoparticles. In a typical synthesis, 2 g of anatase TiO2 particles were added to a 150 mL 10 M NaOH aqueous solution. The specimen was transferred into a Pyrex beaker and statically heated in an oven at 180° C. for 30 h, with the volume of the aqueous solution maintained constant during the heat-treatment process by continual addition of 10 M NaOH solution. The anatase nanotubes could be deposited from solution onto TCO substrates for solar cell use, however, the random orientation of the nanotubes led to the same fundamental trade-off as nanoparticle layers, in that increased thickness for increased light absorption resulted in more charge recombination due to the random walk the photogenerated charge had to make from the nanotubes to the underlying electrical contact.
Ohsaki et al. (Y. Ohsaki, N. Masaki, T. Kitamura, Y. Wada, T. Okamoto, T. Sekino, K. Niihara, S. Yanagida, Dye-sensitized TiO2 nanotube solar cells: fabrication and electronic characterization, Phys. Chem. Chem. Phys., 7 (2005) 4157-4163) synthesized titania nanotubes following the method of Kasuga et al. using P25 TiO2 powder (particle diameter 21 nm) as the starting material. Films of the resulting needle-shaped TiO2 nanotubes were used as the photoanodes in several dye sensitized solar cells, synthesized by spreading a paste of the titania nanotubes upon a transparent conducting glass (fluorine-doped SnO2) substrate. Although the nanotubes were randomly oriented within the photoanode layer, a significant improvement in device photoconversion efficiency was seen in comparison to devices in which the photoanode was comprised of a P25 nanoparticle film of equal thickness (7.1% versus 6.2%). Ohsaki et al. found the titania nanotube electrodes to have longer diffusion lengths and electron lifetimes than those of the P25 nanoparticle films. The longer electron lifetimes result in the titania nanotube devices having higher open circuit voltages, and hence higher photoconversion efficiencies.
A limitation of the technique described by Ohsaki et al. for dye sensitized solar cell fabrication is that the nanotubes comprising the photoelectrode are randomly oriented, so that charge transport within the photoelectrode layer is also, to a significant extent, randomly directed leading to unwanted recombination of the photogenerated charge and hence lower photoconversion, that is sunlight to electricity, efficiencies. Further, the titania nanotube layer comprising the photoelectrode is mechanically applied to the transparent conductive substrate rather than grown directly on the surface resulting in an unwanted series resistance.
Zhao et al. (Y. Zhao, U-H. Lee, M. Suh, Y-U. Kwon, Synthesis and characterization of highly crystalline anatase nanowire arrays, Bull. Korean Chemical Society 25 (2004) 1341-1345) teach synthesis of films comprised of highly ordered crystalline anatase nanowire array. As detailed by Zhao et al., in a typical procedure 0.2 g Ti powder (100 mesh) was added to a 20 mL Teflon vessel followed by addition of 15 mL 10 M NaOH and 1 mL H2O2 (35 wt %). The hydrothermal vessel was placed in a 220° C. oven for 48 h. The reaction produced mat-like films comprised of parallel-aligned sodium titanate nanowires. To convert the sodium titanate nanowires into anatase nanowires, the arrayed sodium titanate nanowire films were submerged into a 1.2 M HCl solution for ion exchange of Na+ with H+. The films were then washed with distilled water to remove the acid, then annealed at 500° C. for 3 h in air to produce pure anatase (TiO2) phase nanowire array films. While there is some uncertainty to the phase of the sodium titanate nanowires, Zhao suggests the overall reactions can be expressed as:Synthesis:6Ti+12H2O2+2NaOH→Na2Ti6O13.xH2O+13H2O  (1)Proton exchange:Na2Ti6O13.xH2O+2H+→H2Ti6O13.x′H2O  (2)Thermal conversion:H2Ti6O13.x′H2O→6TiO2+(1+x′)H2O  (3)Formation of the nanowire array films is intrinsic to the crystal chemistry of alkali titanates and their ion-exchange properties. A limitation of the work of Zhao et al. is that the synthesized nanowire array mats are free-standing, like a self-supporting membrane, rather than synthesized on a transparent conductive-coated glass substrate as needed to achieve a photovoltaic device.
Boercker et al. (J. E. Boercker, E. Enache-Pommer and E. S. Aydil, Growth mechanism of titanium dioxide nanowires for dye-sensitized solar cells, Nanotechnology 19 (2008) 095604) teach growth of randomly oriented polycrystalline anatase TiO2 nanowire films from titanium foil substrates. First, the top surface of the titanium foil is transformed to Na2Ti2O4(OH)2 nanotubes, randomly oriented, through hydrothermal oxidation in a NaOH solution containing H2O2. Next, the Na2Ti2O4(OH)2 nanotubes are converted to H2Ti2O4(OH)2 nanotubes by ion exchange through immersion of the sample in HCl for substitution of the Na+ ions with H+ ions. The H2Ti2O4(OH)2 nanotube samples are then annealed at elevated temperatures for conversion to polycrystalline anatase nanowires through a topotactic transformation. Boercker et al. describe that during annealing the sheets comprising the H2Ti2O4(OH)2 nanotubes, made of edge-bonded TiO6 octahedra, dehydrate and form anatase crystals oriented along the nanotube axis which creates a polycrystalline nanowire.
As described in Boercker et al., TiO2 nanowire films were synthesized in a typical procedure on titanium foil samples approximately 0.127 mm×2.5 cm×2.0 cm in size. The titanium foil samples were placed on the bottom of a 125 ml Teflon-lined hydrothermal pressure vessel with 60 ml 10 M NaOH and 4 ml 35 wt % H2O2. After heating the vessel at 220° C. for 4 h a film of sodium titanate nanotubes, approximately 7 μm thick, formed on the titanium foil surface in contact with the bottom of the vessel. The sample was removed from the vessel, washed with deionized water and dried under flowing Argon. To exchange the Na+ ions with H+ ions, the sample was immersed in 0.57 M HCl for 1 h transforming the sodium titanate nanotubes to hydrogen titanate nanotubes. The sample was removed from the HCl solution, rinsed with deionized water and then dried under flowing Argon. The sample was then heated at 500° C. for 1 h to convert the titanate nanotubes to anatase TiO2 nanowires approximately 20 nm in diameter.
Boercker et al. describe synthesized nanowire films made upon an opaque titanium foil substrate. Such films can only be utilized under so-called ‘back-side’ illumination; in the case of dye sensitized solar cells this requires the incident light to pass through the redox electrolyte before reaching the dye layer where the electrical charge is generated. Since the redox electrolyte absorbs light without charge photogeneration, the resulting device photoconversion efficiencies are always significantly less than those devices illuminated through the electron-transporting TiO2 layer. Further, high temperature annealing of the titanium sample, necessary for the topotactic transformation of the titanate nanotubes to anatase TiO2 nanowires as described by Boercker et al., results in growth of an electrically resistive rutile barrier layer separating the nanowire film from the underlying titanium substrate. When integrated into a photovoltaic solar cell, this rutile barrier layer serves to increase the series resistance of the device in turn decreasing the device photoconversion efficiency. A further limitation is that the nanowires comprising the film are to a significant degree randomly-oriented so that, when used as a photoelectrode, charge transport will be correspondingly randomly directed leading to undesired recombination of the photogenerated charge and hence lower photoconversion efficiencies.
Liu et al. (B. Liu, J. E. Boercker, E. S. Adyil, Oriented single crystalline titanium dioxide nanowires, Nanotechnology 19 (2008) 505604) teach synthesis of single crystal anatase titanium dioxide nanowire array films vertically oriented from a titanium foil substrate. An array of single crystal sodium titanate (Na2Ti2O5.H2O) nanowires is grown on titanium foil through an alkali hydrothermal growth process. An ion-exchange step is used to convert the Na2Ti2O5.H2O nanowires to protonated bi-titanate (H2Ti2O5.H2O) nanowires. A high temperature anneal is used to convert the protonated bi-titanate nanowires to single crystalline anatase TiO2 nanowires through a topotactic transformation. Using this synthesis technique, Liu et al. obtained films comprised of an array of 2-50 μm long single crystal anatase nanowires, perpendicular to the titanium foil substrate, oriented in the [100] direction.
As described in Liu et al., a piece of titanium foil, approximately 0.127 mm×1.5 cm×3 cm in size, is ultrasonically cleaned for 0.5 h in a mixed solution of deionized water, acetone and 2-propanol with volume ratios of 1:1:1. The cleaned titanium sample is placed against the wall of a 125 ml Teflon-lined stainless steel autoclave to which 40 ml of a 1 M aqueous NaOH solution is added. The autoclave is then placed within an oven at 220° C. for durations ranging from 4-60 h, resulting in the growth of single crystal sodium titanate (Na2Ti2O5.H2O) nanowires oriented perpendicular to the titanium foil substrate. The sample is then immersed in 30 ml of 0.6 M HCl solution for one hour to exchange Na+ with H+ ions, resulting in protonated bi-titanate (H2Ti2O5.H2O) nanowires. The sample is removed from the HCl solution and thoroughly rinsed with deionized water or ethanol and dried under ambient conditions. The sample is then furnace annealed at 650° C. for 2 h. The uniform diameter of the individual nanowires indicate that they grow through epitaxial addition of growth units, which Liu et al. suggest are most likely TiO6 octahedra, to the top of the nanowire.
Since the titanium foil substrate effectively represents an infinite source for nanowire source material, Liu et al. teach that increasing the hydrothermal reaction time results in increased nanowire length until depletion of the NaOH solution at approximately 48 h, beyond which continued growth of nanowire length can be achieved with addition of fresh NaOH solution. Increasing the hydrothermal growth time increases both the nanowire length and nanowire diameter. For 1 M NaOH solution a hydrothermal growth time of 6 h results in nanowires 12 μm long and 12 nm diameter; a growth time of 12 h results in nanowires 19 μm long and 26 nm diameter; a 18 h growth time results in nanowires of 22 μm length and 36 nm diameter; a 24 h growth results in nanowires 26 μm long and 47 nm diameter. Hydrothermal growth solutions of higher NaOH molarity results in smaller diameter nanowires.
The nanowire array films described by Liu et al. are single crystal anatase and can be made several tens of microns in length, which are extremely desirable properties for use in dye sensitized solar cells, as well as other 3rd generation solar cell designs such QD and MEG devices. However, those films are grown from an opaque titanium foil substrate. Such films can only be utilized under so-called ‘back-side’ illumination; in the case of dye sensitized solar cells this requires the incident light to pass through the redox electrolyte before reaching the dye layer. Since the redox electrolyte absorbs light without the photogeneration of charge the resulting device photoconversion efficiencies are always significantly less than those devices illuminated through the photoanode. Further, high temperature annealing of the titanium sample, necessary for achieving the anatase TiO2 nanowires, results in growth of a rutile barrier layer separating the nanowire film from the underlying titanium substrate. When integrated into a photovoltaic solar cell, this rutile barrier layer serves to increase the series resistance of the device in turn decreasing the device photoconversion efficiency.
It is desirable to provide an improved self-assembling, vertically-oriented 1-D nanostructure comprised of crystalline structural elements that are synthesized upon a transparent conductive substrate for use in photovoltaic and other applications.